1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a method and system for supporting hard handover in a BWA communication system.
2. Description of the Related Art
In general, wireless communication systems have been developed so as to accommodate a plurality of users using various multiple access technologies. The most typical multiple access technology applied to wireless communication system is a Code Division Multiple Access (CDMA) scheme. The CDMA scheme has evolved from an early voice-oriented communication system into the current high-speed data communication system. The development of the CDMA scheme is attributable to the increasing users' demand for high-speed data transmission and the rapid development of communication technologies. The CDMA scheme has been adopted as a standard for most 3rd Generation (3G) communication systems.
However, since the resources are limited in the CDMA scheme, it is difficult that data rate increases. Nevertheless, nowadays, the data rate required by users is increasing more and more. Therefore, in the wireless communication field, research is being undertaken to support the higher data rates.
For example, the research is being conducted on a communication method using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. The OFDMA scheme forms a plurality of channels using orthogonal frequencies, and allocates at least one channel to each of the users to transmit data. A brief description will now be made of a communication process performed in a communication system using the OFDMA scheme (hereinafter referred to as an “OFDMA communication system”).
The OFDMA communication system uses a scheme for allocating uplink and downlink subchannels. For example, the OFDMA communication system is capable of distinguishing between uplink times and downlink times in a particular time band, and allocating the subchannels to the individual users in the time band. In the OFDMA-based cellular communication system, a method for managing available frequencies can be roughly divided into two methods. The “available frequency management method” refers to a frequency reuse factor management method.
A description will now be made of a first method, which can be considered most typical. The first method uses frequency reuse factors of, for example, 3 or 7, which is greater than 1. The reason for using the high frequency reuse factors will now be described in brief with reference to FIG. 1.
FIG. 1 is a diagram illustrating a frequency reuse scheme in a BWA communication system.
Referring to FIG. 1, base stations (BSs) 100, 110, 120, 130, 140, 150 and 160 form their own cells, and use different frequencies between neighboring cells. For example, one BS uses ⅓ of the total available frequencies. In FIG. 1, each of the BSs uses ⅓ of available carrier indexes. That is, each BS is designed to use only one of the total carrier indexes of n1+n2+n3. For example, the neighboring BSs 110, 120, 130, 140, 150 and 160 of the BS 100 use the frequencies with the carrier indexes unused by the BS 100. By allowing the BSs to use the frequencies having different carrier indexes in this manner, it is possible to efficiently reduce interference between BSs.
When the total available frequencies are divided into 3 groups as shown in FIG. 1, the frequency reuse factor becomes 3. Generally, a BWA communication system, for example, a cellular system, implemented based on the foregoing method has a frequency reuse factor equal to or greater than 3. since it is difficult for all of the BSs to have the ideal cell structure shown in FIG. 1, the frequency reuse factor generally is a value between 3 and 7.
When the frequency reuse factor is a value between 3 and 7, it is impossible to use all of the frequencies. Herein, the number of carrier indexes available in a particular BS is proportional to the possible number of users or a possible data rate. Therefore, a decrease in the number of the carrier indexes restricts the possible number of users or the possible data rate. However, the use of the frequency reuse factor with a value between 3 and 7 contributes to an increase in signal-to-noise (SNR) even at a cell boundary.
Next, a method of using a frequency reuse factor of 1 will be described. In FIG. 1, if the frequency reuse factor is 1, each of the BSs can use frequencies for all of the carrier indexes. As described above, if the frequency reuse factor of 1 is used, frequency resource efficiency increases. However, it causes reduction of a signal-to-interference and noise ratio (SINR) for mobile stations (MSs) located in the cell boundary. That is, when the frequency reuse factor of 1 is used, MSs neighboring their cells have no difficulty in performing communication, but MSs located in the cell boundary suffer performance deterioration or cannot perform communication in some cases.
Due to these problems, in BWA communication system according to the prior art, most discussions were centered on the frequency reuse factor having a value equal to or greater than 3. Recently, however, the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard committee has discussed a method of using a frequency reuse factor of 1.
On the other hand, the communication system introduces the handover concept in order to secure mobility of MSs. The handover enables an MS in communication to seamlessly maintain the communication even through it moves between one BS, i.e., a source BS, and another BS, i.e., a target BS. The handover can be divided into three types of handovers which are known as a soft handover, a softer handover and a hard handover
In the soft handover, while an MS in the source BS moves into the target BS, the MS receives signals from both BSs simultaneously and then eventually connects a call to the desired target BS. The softer handover is similar to the soft handover, but is different in that it is achieved in the same BS. That is, in the softer handover, when an MS moves between sectors in a BS, the BS provides the soft handover to the MS. Therefore, the softer handover is available only for a sectorized BS.
However, the hard handover instantaneously drops the ongoing call to a source BS when an MS in communication moves between BSs, and reconnects the call within the possible shortest time when the MS resumes communication to a target BS.
As described above, research is being undertaken on developing the method of using the frequency reuse factor of 1. In the general BWA communication system, for example, the OFDMA communication system, the hard handover is considered for handover. In this case, MSs located in a cell boundary experience a decrease in SINR during a hard handover, which causes a performance degradation and/or an increased call drop rate. The performance degradation and the increased call drop rate may reduce stability of the communication system. This will now be described with reference to FIG. 2.
FIG. 2 is a diagram illustrating exemplary subchannels individually formed by different BSs in a general BWA communication system. A plurality of orthogonal frequencies for allocating one subchannel in a cell of a particular BS A, and orthogonal frequencies for allocating one subchannel in a cell of another BS B are shown in FIG. 2. Shaded parts in the cell of the BS A represent a plurality of frequencies for allocating one subchannel among the total orthogonal frequencies. Although the OFDMA communication system can sequentially allocate orthogonal frequencies for one subchannel, the OFDMA communication system may generally allocates a plurality of orthogonal frequencies for one subchannel randomly or based on information reported by an MS. Therefore, even in the cell of the BS B, orthogonal frequencies form one subchannel in the same method.
However, when the subchannels individually formed in the cell of the BS A and the cell of the BS B are allocated to, for example, two MSs which are located in the neighboring boundaries of their respective BSs, the two MSs are allocated the same orthogonal frequencies denoted by reference numerals 210 and 220. Serious interference can occurs between the identical orthogonal frequencies 210 and 220 at which collision takes place, causing a reduction in communication quality or a possible drop of the communication.
In addition, when power control is used in the communication system, a user located in the cell boundary transmits data with high power, causing serious interference to users located in other neighboring cells.
As described above, the hard handover causes a low SINR occurring due to the interference resulting from the identical frequencies allocated to users located in neighboring cells. Although the frequency reuse factor is set to a value greater than 1, i.e., to 3 or 7 in order to reduce the interference from the neighboring cells, it can need a special cell planning for allowing neighboring cells or sectors to use different frequencies. In addition, since the neighboring cells cannot use the same frequencies, the frequency efficiency noticeably decreases, giving difficulty in installing new BSs or extending the existing BSs.